Digital Subscriber Line is a new communication technology that allows existing twisted pair Cable Telephone Network to be converted into a high-performance Multimedia Digital Network for multimedia and high-speed data communications with the ability to provide to every subscriber high speed data communication that includes many new services as Video-on-demand, Conference VideoPhone, HDTV Broadcast, Digital Hi Fi Audio, Fast Internet and others.
Digital Subscriber Line technology includes several types of xDSL systems with different patterns of downstream and upstream data bit rate: Symmetrical Digital Subscriber Line (SDSL), Asymmetrical Digital Subscriber Line (ADSL), Very high speed Digital Subscriber Line (VDSL) and others.
These international standards define the frequency bands permitted for downstream (in the direction from a Central Office (CO) of a communication service to a home modem) or of upstream (in the direction from the home modem to the CO) transmission for different DSL systems. As a result, different DSL systems may function properly on the different twisted pairs of the same telephone cable.
FIG. 1 illustrates the frequency band allocation plan for the DSL system in accordance with ITU regulation. A “Plain Old Telephone Service” (POTS) occupies voice frequency band 101 up to a frequency of 4 kHz. An Asymmetric Digital Subscriber Line (ADSL) uses digital multitone (DMT) line signals for communication between subscriber equipment and the central office (CO) of the telephone station. ADSL equipment of the CO transmits downstream data by modulation of about 200 tones in frequency band 105 from 200 kHz to 1100 kHz and receives about 30 tones of upstream data in frequency band 103 from 28 kHz to 140 kHz. ADSL systems are able to transmit downstream data with speeds of up to 10 Mb/s and upstream data with speeds of up to 1 Mb/s. ADSL was specially developed for long cable lines of up to 4.5 km in length.
In VDSL, data may be transmitted with a much higher bit rate: downstream data up to 56 Mb/s and upstream data up to 26 Mb/s but works only on short cable lines of up to 1.5 km.
As shown on FIG. 1 four frequency bands are defined for the VDSL system: two for downstream transmission (one band 107 from 0.3 MHz to 3.0 MHz and the other band 109 from 5.2 MHz to 7.5 MHz), and two bands for upstream transmission (one band 111 from 3.0 MHz to 5.2 MHz and another band 113 from 7.5 MHz to 12 MHz). Depending on cable length and necessary bit rate these bands may be partially or completely used. VDSL may be realized with DMT line signal and with CAP modulation.
The most wide-spread ADSL use is in point-to-point connected circuits. Such a system includes an ADSL modem on each end of the twisted pair telephone line, creating three information channels—a high speed downstream (central office to end user) channel, a medium speed upstream (end user to central office) channel, and a POTS (“Plain Old Telephone Service”) channel. The POTS channel is separated from the ADSL modem by filters, thus guaranteeing uninterrupted POTS, even if the ADSL circuit fails.
Two variants of ADSL systems are available today: full-rate ADSL in accordance with the T1E1.413 or ITU G.992.1 standards and “splitterless” ADSL in accordance with the ITU G.992.2 standard. Full-rate ADSL uses POTS splitters to separate the POTS channel from the ADSL data signals. A POTS splitter is installed at each end of the line and includes a low-pass filter for separating out POTS telephone voice communication signals and a high-pass filter for separating out data communication signals.
The POTS splitter divides the subscriber line into two separate twisted pairs—one for data communication (ADSL) and one for telephone voice communication signals (POTS). As a result, the existing two-wire internal house telephone wiring is not usable for ADSL. New wiring must be installed from the splitter to the modem, resulting in increased installation cost.
Splitterless ADSL can be installed without the need for additional home wiring. In this case, the ADSL modem includes a high-pass filter that rejects the POTS telephone voice communication signal, while every telephone instrument in the house is connected to the telephone line through a low-pass microfilter that rejects the ADSL data signals. A splitterless ADSL system is advantageous in that it is a “Plug and Play” system. A customer does not need any special service from his telephone company to install the ADSL home equipment. What is needed therefore is an ADSL home modem and a number of microfilters (according to the number of telephones in the house) and to plug those devices into existing telephone connectors. Given the ease of connection several PC (personal computer) manufacturers include splitterless ADSL modems (G.LITE modems) within the PCs motherboards and also supply the several microfilters with the PC.
FIG. 2 is a block diagram illustrating a prior art splitterless ADSL system generally designated 201. A number of subscriber premises 203 are coupled to a central office (CO) 209 by a telephone cable 217 comprising a plurality of twisted pair subscriber telephone lines 207. At the subscriber premises 203 there are customer premises equipment including personal computers (PC) 225 (two in this specific example), a printer 227, a fax 221 and telephone devices 223. One PC 225 is connected to another PC 225 and to printer 227 by parallel port cables 229 constituting together a digital home network. An ADSL modem is connected directly to telephone line 207 and by an Ethernet cable 224 to one of the PCs 225. Fax 221 and telephone device 223 are connected to telephone line 207 by microfilters 219. CO 209 includes ADSL Office Equipment 211, a data switch 235, linked to a data network 215 and a voice switch 237 linked to a voice network 213. Each twisted pair subscriber telephone line 207 is coupled to a POTS splitter 231, which is coupled to the voice switch 237 and to an ADSL office modem 233.
Voice communications passing through voice switch 237 are passed through POTS splitter 231 and applied to twisted pair 207 as baseband signals. Data communications passing through data switch 235 are modulated at a frequency range higher than that of the baseband POTS signals, passed through POTS splitter 231 and applied to twisted pair 207. Since the data communications are transmitted at a different frequency range than the voice communications, frequency-division multiplexing (FDM) allows simultaneous transmission of both voice communications (POTS) and data communications over a single twisted pair 207.
A standard ADSL system has some shortcomings. For one, an ADSL system needs to be always connected. It means that the ADSL modem continuously transmits and receives DMT signals regardless whether information is transmitted or not. As a result, a plurality of ADSL modems 233 of the Central Office work continuously and consume extensive energy. Additionally, at the other end, only one ADSL home modem may be connected to the telephone line in the home because frequency bands of upstream and downstream are always busy. In the USA, this has become a problem as about 20 million subscribers have more than one PC inside the home. When two or more PCs in the home have internal ADSL modems, only one of them may be physically connected to the telephone line. Every time a customer wants to access the Internet from another PC, he must connect this PC to the telephone line and disconnect other PCs.
Another problem is that existing splitterless ADSL systems do not support communication between several computers within the home. It means that a subscriber must have an additional home data network like Ethernet and additional network equipment inside their PCs. A further problem resides in that existing ADSL home modems may not support future VDSL systems. A VDSL system is expected to become widespread over the coming years. Telecommunications companies are expected to replace existing ADSL Central Office equipment with new VDSL equipment, which will be able to support a much higher data bit rate.